The present invention relates to a catalyst recovery process. More particularly, the present invention relates to a process for the removal of fluoride compounds from spent alkylation catalysts comprised of fluorosulfonic acid and sulfuric acid, and to the regeneration of fresh alkylation catalyst.
Liquid phase alkylation processes wherein an isoparaffin hydrocarbon, such as isobutane, isopentane, etc. are alkylated with olefin hydrocarbons such as propylene, butylenes, etc. for the production of alkylate products comprising highly branched C.sub.7 - C.sub.8 range paraffin hydrocarbons having high octane value, are well known and widely practiced. In such processes, the reactant hydrocarbons are usually contacted in the liquid phase, at elevated temperatures in the presence of acid alkylation catalysts and under conditions of good mixing, reaction pressures usually being only sufficient to maintain the reactants in the liquid phase.
Although numerous acid catalysts may be employed in such alkylation processes, an effective catalyst comprises a mixture of sulfuric acid and fluorosulfonic acid. One such catalyst, particularly effective in the alkylation processes under consideration is disclosed in co-pending United States Patent Application Ser. No. 520,595, filed Nov. 4, 1974. The novel alkylation catalyst disclosed in the aforementioned application comprises fluorosulfonic acid and sulfuric acid in a weight ratio of from about 0.11/1 to about 0.32/1, respectively, the catalyst having a titratable acidity in the range of 16.6 to 21 milli-equivalents per gram (meq/gm) and which may contain up to 3% by weight water and up to 10% by weight acid oils, the acid oils comprising relatively high molecular weight reaction products of sulfuric acid and hydrocarbons present in the process. In the process described in the aforementioned patent application, a C.sub.4 - C.sub.6 isoparaffin hydrocarbon such as isobutane is contacted with a C.sub.3 - C.sub.5 olefin hydrocarbon such as propylene, a butylene, or mixtures thereof, in a molar ratio of isoparaffin to olefin of from about 2/1 to 20/1, in the liquid phase, in the presence of the alkylation catalyst at a temperature in the range of from about 0.degree.F. to about 100.degree.F. Reaction pressures employed may range from ambient to superatmospheric, the pressure employed generally being sufficient to maintain the hydrocarbon reactants in the liquid phase. Since the reactants may be normally gaseous at alkylation reaction temperatures, reaction pressures generally range from about 10 to about 150 psig. Preferably, the alkylation reaction mixture is subjected to good mixing to form a hydrocarbon in continuous acid phase emulsion which comprises from about 40 to about 70 volume % acid phase and from about 60 to about 30 volume % hydrocarbon phase. Liquid volume ratios of isoparaffin hydrocarbons to olefin hydrocarbons of from about 2/1 to about 20/1 are generally employed in the process. Contact times for hydrocarbon reactants in the alkylation zone, in the presence of the alkylation catalyst, may range from about 0.5 to about 60 minutes. Preferably, the contact time is sufficient to ensure essentially complete conversion of olefin reactant in the alkylation zone. Such contact times are sufficient for providing an olefin space velocity in the range of about 0.1 to about 1.0 volumes olefin/hour/volume of catalyst. The process may be conducted batchwise or continuously. It has been found that use of the catalyst described in the aforementioned patent application, in alkylating C.sub.4 - C.sub.6 isoparaffin with a C.sub.3 - C.sub.5 monoolefin, produces an alkylate of increased octane value over that obtained by prior art catalysts.
When using the above-described fluorosulfonicsulfuric acid alkylation catalyst, or for that matter, any sulfuric acid alkylation catalyst containing a similar fluoro acid, it is common practice to process the spent catalyst in such a fashion so as to regenerate fresh sulfuric acid, the major component of the catalyst. However, even though the fluorosulfonic acid is present as a minor component in the alkylation catalyst, because of its expense relative to that of sulfuric acid, it is desirable to recover, as well, the fluorosulfonic acid or any fluoro compounds which can be easily converted to the fluorosulfonic acid.
One method for recovering the sulfuric acid from the spent alkylation catalyst is to treat the spent catalyst in what is known as a sludge conversion unit. In such a unit the spent catalyst containing water and organic materials is charged to a furnace for oxidative conversion of all the sulfur species present to sulfur dioxide. The sulfur dioxide, in admixture with air, is then passed over a catalyst, e.g. V.sub.2 O.sub.5 or some other such suitable oxidation catalyst, in a converter section of the unit to form SO.sub.3. The SO.sub.3 is then absorbed in a sulfuric acid solution to produce oleum which is then diluted with water to produce sulfuric acid of the desired concentration, i.e. 97 - 99 weight %, for the alkylation catalyst. The furnaces used in such sludge conversion units employ refractories which are readily attacked by HF or HF precursors such as fluorosulfonic acid. For example, to prevent damage to refractory furnace linings, fluoride concentrations (calculated as HF) of about 10 ppmw or less are particularly desirable. At levels above about 10 ppmw, fluoride attack upon refractory lining is accelerated, thus shortening the operating lifetime of such materials in the sludge conversion unit. Over and above the potential damage to the refractories in the furnace, any HF in the converter section would volatilize the vanadium from the V.sub.2 O.sub.5 oxidation catalyst. Accordingly, a process which effectively recovers the fluorosulfonic acid or precursors thereof from the spent catalyst and also provides a feed to the sludge conversion unit substantially free of damaging fluoro compounds is highly desirable.